First analysis of Earth-sized planet's atmosphere reveals surprises

Scientists have for the first time analyzed the atmospheric composition of a distant planet roughly the size of our own, revealing a mix of hydrogen, helium and carbon-based molecules(Credit: NASA/ESA Hubble Space Telescope)

Using instrumentation aboard the Hubble Space Telescope, astronomers have been able for the first time to detect and analyze the atmospheric composition of a distant super-Earth (a planet roughly the size of our own). The findings show that the planet's atmosphere is rich in hydrogen, carbon and helium, giving scientists important clues as to how such planets might have formed and evolved.

Discovered in 2004, 55 Cancri e is classed as a "super-Earth," a category comprising planets with a mass between one and 10 times that of our own pale blue dot. 55 Cancri e, for instance, is eights times as massive and twice the diameter of Earth.

Super-Earths are believed to be the most common type of planets in our galaxy, but there are least three highly unusual factors that make a rare and precious find out of 55 Cancri e. Firstly, the planet is only a modest (astronomically speaking) 40 light-years away from Earth; secondly, it orbits very close to its star, which raises its surface temperature into the thousands of degrees and makes it visible in the infrared; and lastly, it crosses directly in front of its sun once every 18 hours from Earth's vantage point, making it possible to observe it in the visible spectrum too.

Previous studies on 55 Cancri e have led astronomers to nickname it the "diamond planet" because its core is believed to be rich in carbon, which would take the form of diamond under the planet's great pressure. Later investigations from the Spitzer space telescope led to speculations that Cancri e also featured water on its surface at temperatures and pressures so high that would melt away the planet's rocks and generate thick layers of steam, though nothing could be said of the planet's atmosphere.

Astronomers can infer the mass and diameter of distant planets with relative ease, but analyzing the composition of their atmosphere is much more challenging. Back in 2014, when Hubble's Wild Field Camera 3 (WFC3) was pointed at two super-Earths, the resulting data was vague and could only suggest that the planets were wrapped in thick clouds of unidentified molecules much heavier than hydrogen.

But now, a new analysis technique and the unique, favorable characteristics of 55 Cancri e have revealed no steam (as was previously believed) but instead large amounts of hydrogen and helium in the planet's atmosphere.

"This is a very exciting result because it's the first time that we have been able to find the spectral fingerprints that show the gases present in the atmosphere of a super-Earth," explains PhD student Angelos Tsiaras, who helped develop the new analysis technique. "The observations of 55 Cancri e's atmosphere suggest that the planet has managed to cling on to a significant amount of hydrogen and helium from the nebula from which it originally formed."

The data also hinted at the presence of hydrogen cyanide, which would indicate a carbon-rich atmosphere and a high ratio of carbon to oxygen, with possible additional presence of such molecules as carbon monixide (CO), carbon dioxide (CO2) and acetylene (C2H2).

"This result gives a first insight into the atmosphere of a super-Earth," said Giovanna Tinetti, who was part of the research team. "We now have clues as to what the planet is currently like and how it might have formed and evolved, and this has important implications for 55 Cancri e and other super-Earths."

The short video below shows an artist's illustration of the planet's atmosphere.